The present invention relates to a tube expander for fixing tubes to a tube plate of a boiler or multitubular heat exchanger and, more particularly, to a tube expander relying upon a radial expansion of a cylindrical elastic expanding medium inserted into a tube caused by an axial compression of the medium for expanding and fixing the tube to the tube-receiving bore of the tube plate.
In manufacturing boilers and multitubular heat exchanger or the like, such a method of fixing tubes to a tube plate is getting popular as consisted in inserting a cylindrical elastic expanding medium and compressing the medium in the axial direction to cause the later to exert a radical expanding force which in turn acts on the inner peripheral wall of the tube to radially expand and tightly fix the tube to the tube plate.
Conventionally, tube expanders for carrying out the above explained method incorporates a cylindrical elastic expanding medium, a pressurizing rod passing through the medium and a back-up ring adapted to be secured straddling a tube-receiving bore of the tube plate. The pressurizing rod is connected to a piston rod in a hydraulic cylinder. In operation, the hydraulic cylinder exerts a force on a rod-head of the pressurizing rod so that the rod effects an axial compression and a consequent radial expansion of the expanding medium. The radial expansion of the medium in turn expands the tube radially and tightly fits it to the tube-receiving bore of the tube plate.
In those conventional tube expanders, as the tube is radially expanding, a gap left between the inner surface of the tube and a central boss of the back-up ring is also enlarged. Thus, part of the expanding medium inconveniently invades this enlarged gap causing a collapse or deformation of the expanding medium. This unfavorable invasion of the expanding medium takes place also at the end of the pressurizing rod opposite to the back-up ring, where a gap is formed between the rod-head of the pressurizing rod and the tube.
In order to prevent this undesirable collapse of the expanding medium, it has been proposed to put a cylindrical seal ring having parallel end surfaces between the expanding medium and the tube. This seal ring is adapted to expand radially, when compressed axially, to increase its diameter thereby to prevent the collapse of the expanding medium.
The radial deformation or displacement of the seal ring depends on the hardness of the material of the seal ring. A soft seal ring would be collapsed as it is pressed onto the back-up ring, although it may exhibit a large radial displacement to ensure a larger sealing effect. Therefore, the material of the seal ring is selected to have a larger hardness than the expanding medium.
The seal ring is, however, not effective when the expanding pressure reaches 3000 to 4000 kg/cm.sup.2 as is the case where a large airtightness and a large fixing force is required between the tube and the tube plate, although it can do pretty well for thin tubes. Namely, the larger radial displacement of the seal ring for ensuring the larger sealing effect and the prevention of the collapse of the seal ring are incompatible with each other, since the collapse prevention is ensured only through an enhanced hardness which provides a poor radial displacement.
Under these circumstance, the invention is aiming at overcoming the drawbacks of the prior art by providing an improved tube expander which is free from the collapse of the expanding medium even at a large expanding pressure.
According to the invention, there is provided a tube expander comprising an elastic expanding medium adapted to be received by a tube to be expanded, a pressurizing rod passing through the expanding medium, a rod-head provided at one end of the pressurizing rod, a back-up ring slidingly passed by the pressurizing rod and seal rings made of an elastic material of a hardness greater than that of the expanding medium disposed close to both ends of the expanding medium and slidingly passed by the pressurizing rod, characterized in that a conical recess is formed in at least one seal ring at a portion thereof facing one end surface of the expanding medium.
The described and other objects, as well as the advantageous features of the invention will become clear from the following description of preferred embodiments taken in conjunction with the attached drawings in which: